Accumulating evidence from single cell recording studies in primates indicates that neurons encode spatial information in a variety of reference frames, which may be transformed to a single, common viewer (eye-) centered representation in posterior parietal cortex used for planning movements. Functional imaging studies also indicate that posterior parietal cortex is crucial for localization of objects in space (essential for planning saccades or reaching) in humans, whereas temporal cortex is crucial for object and word recognition. Lesions studies in both monkeys and humans indicate that spatial representations with different reference frames can occasionally be separately disrupted by brain damage, variously impairing action or perception on the side contralateral to the brain lesion (causing different forms of unilateral spatial neglect, or USN). However, the cortical organization of such spatial representations is fundamentally different in primates vs. humans, as indicated by the fact that lesions in either hemisphere cause comparable USN in monkeys, whereas right cortical lesions cause more common, severe, and persistent USN in humans. Conflicting results regarding the neural correlates of various types of USN in humans have been reported, which may be due to limitations of chronic lesion-deficit correlation studies. The goal of this project is to identify the nature and the neural basis of different types of USN with a novel approach utilizing MR perfusion weighted imaging (PWl) and diffusion weighted imaging (DWl), along with testing of USN at the same time, in subjects <24 hours post onset of stroke, and at three days after onset. This method complements functional imaging studies and avoids many of the pitfalls of chronic lesion-deficit studies. PWI shows regions of hypoperfused, dysfunctional tissue, while DWl reveals densely ischemic or infarcted tissue, in acute stroke. The major hypothesis is that PWl and DWl with concurrent USN testing can reveal areas of neural dysfunction, with or without structural damage, associated with USN in different reference frames or in different tasks, before substantial reorganization. It is further hypothesized that restoration of tissue function in a given region (by restoring regional blood flow) will result in resolution of specific types of USN. Results are expected to reveal distinct areas of cortex crucial for computing spatial representations with different reference frames, and to reveal mechanisms of USN in tasks with different purposes.